Cable modems receive signals from a server, such as video signals and other data on a downstream portion of a frequency spectrum. The server is typically called a cable modem termination system (CMTS), which is coupled to the cable modems via a hybrid fiber and coaxial cable (HFC) network. The downstream portion is typically in the range with a lower frequency of 108 to 258 MHz and an upper frequency of 1218 to 1794 MHz. The cable modems also provide data back to the server on an upstream portion of the frequency spectrum. The division of the spectrum is referred to as frequency division duplex, which utilizes different portions of the spectrum for upstream and downstream communications, which when used simultaneously is referred to as full duplex. The upstream portion has a frequency range that is typically 5 Mhz to 85 or 204 MHz, depending on the downstream portion of the spectrum being used. The upstream portion and downstream portion of the spectrum may be simultaneously used for transmissions downstream and upstream.
In full duplex cable modem systems, there can be interference between different modems. To minimize the interference, constraints have been placed on the cable modems, such as a modem receiving on a channel cannot transmit on that channel, but other sufficiently radio frequency (RF) isolated modems may transmit on that channel. Intelligent scheduling by the CMTS may also be utilized to minimize such interference. Still further, cable modems may be assigned to various transmission groups to minimize interference.
Protocols for implementing communications in cable systems are described in a DOCSIS® family of specifications developed by Cable Television Laboratories (CableLabs). The family of specifications defines a fifth generation of high-speed data-over-cable systems, commonly referred to as the DOCSIS 3.1 specifications.